[Mapserver-users] Can't seem to get the projection correct
Paul Ramsey
pramsey at refractions.net
Tue Feb 18 16:53:54 EST 2003
I think he means just plain old proj=merc, the classic projection from
good old Mercator himself.
+proj=merc +lat_ts=Latitude of natural origin
+lon_0=Longitude of natural origin
+k_0=Scale factor at natural origin
+x_0=False Easting
+y_0=False Northing
Martin, Daniel wrote:
> Thanks Ed, I'm tinkering with Mercator now (or at least I think I
> am). Not having much luck actually - but I think you are right. I'm
> just not very experienced with projections, so working with them is a
> matter of trial and error for me. I'm assuming you mean Mercator as
> in proj=tmerc not proj=utm, though I wouldn't know better either way.
> I've basically been trying options from the PROJ list,
> http://mapserver.gis.umn.edu/doc36/proj.html
>
> Most of the time I end up with an error, a blank map, or a map that
> looks like a rolled newspaper. I'll get it right eventually.
>
> Thanks again, Dan
>
>
>
>
>> -----Original Message----- From: Ed McNierney
>> [mailto:ed at topozone.com] Sent: Tuesday, February 18, 2003 2:49 PM
>> To: Martin, Daniel; mapserver-users at lists.gis.umn.edu Subject: RE:
>> [Mapserver-users] Can't seem to get the projection correct
>>
>>
>> Daniel -
>>
>> As I mentioned before, you are probably accustomed to seeing maps
>> in the Mercator projection. Rather than fiddling with the image
>> size, you might want to look into having MapServer generate
>> Mercator output images.
>>
>> The Mercator projection stretches the Y-dimension as compared to
>> the geographic projection. The distance between parallels of
>> latitude increases as you move away from the equator. At a
>> Missouri-ish latitude of 40 degrees North, one degree of latitude
>> is 1.414 times taller in a Mercator projection than in a geographic
>> projection. This would mean that if your horizontal (longitude)
>> scale were, say, 7 pixels per degree, you would expect your
>> vertical (latitude) scale to be 7 * 1.414 = 9.9 pixels per degree.
>>
>> Since this happens to be exactly what we see in the MapInfo image
>> you sent (which is roughly centered on Missouri), I think it's
>> pretty likely that both your memory and your MapInfo maps are in
>> the Mercator projection. If you set up MapServer to create
>> Mercator output images I suspect you'll match your MapInfo results
>> without having to fudge the image size (which isn't really correct
>> anyway because the vertical scale distortion is nonlinear and
>> varies with the latitude).
>>
>> - Ed
>>
>> Ed McNierney President and Chief Mapmaker TopoZone.com / Maps a la
>> carte, Inc. 73 Princeton Street, Suite 305 North Chelmsford, MA
>> 01863 Phone: (978) 251-4242 Fax: (978) 251-1396 ed at topozone.com
>>
>>
>> -----Original Message----- From: Martin, Daniel
>> [mailto:DMartin at erac.com] Sent: Tuesday, February 18, 2003 3:17 PM
>> To: mapserver-users at lists.gis.umn.edu Subject: RE:
>> [Mapserver-users] Can't seem to get the projection correct
>>
>> I want to thank everyone for all the responses.
>>
>> Daniel Morissette hit the nail on the head. MapInfo and Mapquest
>> are indeed doing some matter of x-scale adjustment to make maps
>> "look better".
>>
>> I know this isn't scientific, but my maps in MapInfo look like I
>> expect them too. For instance, my home state of Missouri that I've
>> seen on maps thousands of times "looks" like I know it to look in
>> MapInfo. In MapServer it "looks squished". I don't know how to
>> express it any different than that. One way I've adjusted for it
>> is to force a new aspect ratio in the <img > tag on the HTML
>> template to "squish" the image back. But I hate that solution.
>> It's not a good solution in my book.
>>
>> I'm going to take a look into some things mentioned and see if I
>> can get anywhere. I'll be sure to let you know how they turn out.
>>
>> Thanks again, Dan Martin
>>
>>
>>
>>
>>> -----Original Message----- From: Daniel Morissette
>>> [mailto:morissette at dmsolutions.ca] Sent: Tuesday, February 18,
>>> 2003 12:10 PM To: Martin, Daniel Cc:
>>> mapserver-users at lists.gis.umn.edu Subject: Re: [Mapserver-users]
>>> Can't seem to get the projection correct
>>>
>>>
>>> "Martin, Daniel" wrote:
>>>
>>>> Sorry to correct you, but that's not true - at least not
>>>
>>> since I've worked
>>>
>>>> with MapInfo (version 5 through 7). That may be an optional
>>>
>>> configuration,
>>>
>>>> but by default MapInfo doesn't change the aspect ratio of
>>>
>>> the data with the
>>>
>>>> window. By default, the data's horizontal extent stays the
>>>
>>> same as the
>>>
>>>> window is resized, but the vertical changes to maintain the
>>>
>>> projection.
>>>
>>> You're correct, I was wrong on that, I should have checked first.
>>> I found the option that's causing this effect... open your
>>
>> lat/lon map,
>>
>>> then select Map/Options, the "Distance/Area Using:" radio box is
>>> set to Spherical. Change the projection to "Non-Earth", and the
>>> "Distance/Area" will switch to cartesian, and then your map
>>
>> will look
>>
>>> exactly as it does in MapServer.
>>>
>>> I'm not sure what this spherical distance option does exactly but
>>> it seems that MapInfo (and MapQuest) use this to adjust the
>>> x-scale of the map to make it look better, that's why the map
>>> looks different, this is not a projection effect, it's a map
>>> scaling effect.
>>
>> Unfortunately at
>>
>>> this point MapServer supports only maps with x-scale == y-scale
>>> (or square pixels if you like) so you won't be able to reproduce
>>> that unless you can define a projection that produces a similar
>>> effect.
>>>
>>> -- ------------------------------------------------------------
>>> Daniel Morissette morissette at dmsolutions.ca DM
>>> Solutions Group http://www.dmsolutions.ca/
>>> ------------------------------------------------------------
>>>
>>
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>> http://lists.gis.umn.edu/mailman/listinfo/mapserver-users
>>
>
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| Paul Ramsey
| Refractions Research
| Email: pramsey at refractions.net
| Phone: (250) 885-0632
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